Packers are widely used in boreholes to isolate a portion of the borehole from another. Some of these packers are set with tubing pressure that either inflates an element or operates a piston to axially compress an assembly of a sealing element and adjacent slips. This is commonly accomplished with a ball dropped on a ball seat so that pressure above the seated ball is communicated to a piston outside the string through a wall opening. The applied pressure breaks any retainers on piston movement and as a result an axial compressive force acts on the seal and slips to set the packer. In other designs the available hydrostatic pressure is used as the driving force to move a piston to in turn set the seal and the slips of a packer. In still other designs the tubular string associated with the packer is manipulated to set the packer.
There are disadvantages to some of these designs. One notable disadvantage is the need to have a wall opening in designs that set the packer with internal tubing pressure. For the packers that set hydrostatically with annulus pressure the can still be wall openings to an exterior piston that opens a port to allow access of annulus pressure to a piston to set the packer. Another technique involves signaling a valve to open at the packer in the annulus from the surface through a variety of techniques such as coded pressure pulses, vibration or movement patterns of a work string. Each of these techniques has disadvantages of cost or limited applicability due to well conditions. The techniques for remote signaling require a local processor and signal receiver.
In some hydrostatically set packers rupture discs have been suggested to provide a backup way to communicate annulus pressure to a piston that would set the packer. As an alternative to a rupture disc 42 U.S. Pat. No. 6,779,600 suggested a disappearing plug to provide a time delay to providing annulus hydrostatic pressure access to the operating piston of the packer. The lock sleeve 32 had its own mechanical restraint in shear pin 46. Movement of the lock sleeve 32 released dog 48 from groove 50 to allow hydrostatic pressure to actuate the packer by moving piston 18 against an atmospheric chamber 24. Breaking the rupture disc 42 or having a plug dissolve let in hydrostatic pressure to break the shear pin 46 to liberate piston 18 to set the packer. This design still depended on a shear pin to break at a designated force and to shear cleanly to allow the parts to relatively move thereafter.
Another design shown in US Publication 2012/0279701 FIG. 8 shows the use of shape memory alloy for plug 202 that is thermally induced to go into another shape to open passage 200 so that hydrostatic pressure moves the piston 206 to break a shear pin that holds ring 210 to release the lock 212 on packer mandrel 214 that allows setting the packer with pipe manipulation and drag blocks. Paragraph 26 alludes to an option to retain a preload force on a piston with a member that is dissolved or chemically attacked to release the force to move the piston. No drawing of this alternative is provided.
Controlled electrolytic materials (CEM) have been described in US Publication 2011/0136707 and related applications filed the same day. These materials dissolve in well conditions.
Also relevant to disappearing plugs are US Publication 2012/0118583; U.S. Pat. No. 7,552,777 (swelling material shifts a sleeve to open a port) and U.S. Pat. No. 7,726,406 (FIG. 4 where core 47 of plug 43 disappears and puts a force on a piton 41 to break retaining shear pin 42 to set the tool.
The above locking mechanisms are all indirect techniques for retaining an actuator that still depend on shear pins and the uncertainties that are involved in their use. The present invention incorporates the mechanical locking member for a tool actuator as the part that goes away so that the tool can be set. More specifically in a hydrostatically set packer has a CEM locking member that dissolves to allow the packer to set. These and other aspects of the present invention will be more readily apparent from a review of the description of the preferred embodiment and the associated drawings while understanding that the full scope of the invention is to be determined from the appended claims.